Polypeptides with improved arabinose transport specificity

ABSTRACT

The invention relates to a variant of a parent polypeptide which is preferably a hexose transporter, wherein the variant comprises an amino acid sequence which, when aligned with the amino acid sequence of SEQ ID NO: 1, comprises a substitution of amino acids N376 and T89, the positions of said amino acids being defined with reference to the amino acid sequence of SEQ ID NO: 1. Said polypeptide is suitable in the production of ethanol from arabinose-rich biomass such as corn stover and corn starch.

FIELD

The invention relates to variant polypeptides with improved arabinose transport specificity and reduced glucose transport activity, a recombinant yeast comprising a nucleic acid sequence encoding said polypeptides, and their use to produce a fermentation product such as ethanol.

BACKGROUND

By functionally replacing fossil-fuel derived compounds, microbial production of chemicals and transport fuels can contribute to a transition to a sustainable, low-carbon global economy. The total industrial production of fuel ethanol, which reached ca. 100 billion liters in 2015, is predicted to increase further. The yeast Saccharomyces cerevisiae is the established microbial cell factory for conversion of starch and sucrose derived hexose units to ethanol, as it combines a high ethanol yield and productivity with robustness under process conditions. Efforts in yeast strain improvement and process optimization of corn-starch and cane sugar based bioethanol production have further improved product yields and productivity. Furthermore, intensive metabolic and evolutionary engineering studies have yielded yeast strains capable of efficiently fermenting the pentose sugars xylose and arabinose, thus paving the way for yeast based ‘second-generation’ bioethanol production from lignocellulosic hydrolysates.

The hexose transporter Gal2 is known to mediate arabinose transport (Becker J, Boles E. 2003, A modified Saccharomyces cerevisiae strain that consumes L-Arabinose and produces ethanol, Appl Environ Microbiol. 69: 4144-4150). However, the affinity for the respective pentose sugars is approximately 10 to 100 times lower than for the respective hexose sugars. The lack of a dedicated xylose or arabinose transporter in recombinant yeast cells thus limits the capacity for co-utilization of hexoses and pentoses in sugar mixtures, and prohibits a high pentose catabolic flux. As a consequence, conversion of biomass sugars may be considered bi-phasic: in the first phase, a relatively fast conversion of hexoses (glucose) takes place, while in the second phase, which starts when the hexoses have been exhausted from the medium, pentose fermentation commences, but at a far lower rate as compared to the rate of hexose conversion.

It is therefore a long-felt desire to express arabinose specific sugar transporters, i.e. no glucose interference (arabinose specificity) and high affinity to arabinose in order to maintain the ability to convert hexoses at approximately the same level.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Uptake experiments using 50 mM [140-] L-arabinose in the presence of a range of D-glucose concentrations using strain DS68625 expressing Gal2 variants: N376T+T89I (open circles), N376T (closed circles), T89I (open squares) and Gal2 wild-type (closed squares). FIG. 2. Uptake experiments using 50 mM [¹⁴C-] L-arabinose in the presence of a range of D-glucose concentrations using strain DS68625 expressing Gal2 variants: N376T+T89I (open circles), N376T (closed circles), T89I (open squares) and Gal2 wild-type (closed squares).

FIG. 3. Uptake experiments using various concentrations of [¹⁴C-] L-arabinose (top) or [140] D-glucose (bottom) to determine K_(m) and V_(max) of strain DS68625 expressing Gal2 variants: T89I (open circles), N376T+T89I (closed circles), N376T (open squares), Gal2 wild-type (closed squares), N376S (closed diamonds) and N3761 (open diamonds).

DETAILED DESCRIPTION

An object of the invention is to provide novel variant hexose transport polypeptides having reduced glucose affinity. Another object of the invention is to provide novel variant hexose transport polypeptides having high affinity for arabinose. One or more of these objects are attained according to the invention.

Throughout the present specification and the accompanying claims, the words “comprise” and “include” and variations such as “comprises”, “comprising”, “includes” and “including” are to be interpreted inclusively. That is, these words are intended to convey the possible inclusion of other elements or integers not specifically recited, where the context allows. The articles “a” and “an” are used herein to refer to one or to more than one (i.e. to one or at least one) of the grammatical object of the article. By way of example, “an amino acid” may mean one amino acid or more than one amino acid.

Amino acid sequence aligned with the amino acid sequence set out in SEQ ID NO: X (when referring to a variant polypeptide) means that the variant amino acid sequence and the amino acid sequence set out in SEQ ID NO: X are aligned by a suitable method which allows comparison of the sequences with each other and identifications of the positions in the amino acid sequence of the variant wherein either the same amino acid is present (identical position), or another amino acid is present (subsitution), or one or more extra amino acids are present (insertion or extension) or no amino acid is present (deletion or truncation) if compared with the amino acid sequence set out in SEQ ID NO: X.

A suitable method allowing comparison of two amino acid sequence may be any suitable Pairwise Sequence Alignment method known to those skilled in the art, preferably a Global Pairwise Sequence Alingment method. A preferred Global Pairwise Sequence Alignment method is the EMBOSS Needle method based on the Needleman-Wunsch alignment algorithm (aiming at finding the optimum alignment (including gaps) of the two sequences along their entire length) (Needleman, S. B. and Wunsch, C. D. (1970) J. Mol. Biol. 48, 443-453) as described herein. In one embodiment the amino acid sequence is aligned with the amino acid sequence set out in SEQ ID NO: x using the NEEDLE program from the EMBOSS package, using EBLOSUM62 as a substitution matrix, with a gap-open penalty of 10 and a gap extension penalty of 0.5.

A “yeast” or “yeast cell” as defined herein is a yeast suitable for genetic manipulation and which may be cultured at cell densities useful for industrial production of a target product. A yeast or yeast cell may be found in nature or a cell derived from a parent cell after genetic manipulation or classical mutagenesis.

The term “control sequence” as used herein refers to components involved in the regulation of the expression of a coding sequence in a specific organism or in vitro. Examples of control sequences are transcription initiation sequences, termination sequences, promoters, leaders, signal peptides, propeptides, prepropeptides, or enhancer sequences; Shine-Delgarno sequences, repressor or activator sequences; efficient RNA processing signals such as splicing and polyadenylation signals; sequences that stabilize cytoplasmic mRNA; sequences that enhance translation efficiency (e.g., ribosome binding sites); sequences that enhance protein stability; and when desired, sequences that enhance protein secretion.

The term “culturing” refers to a method of multiplying microorganisms in a nutrient medium and under conditions suitable for the growth and/or propagation of said microorganism and/or the production of a compund of interest by the microorganism. These methods are known in the art. When the microorganism is able to express/produce a compound of interest, for example, the microorganisms may be cultured by shake flask culturing, small-scale or large-scale fermentation (including continuous, batch, fed-batch, or solid state fermentations) in laboratory or industrial fermenters performed in a suitable medium and under conditions allowing the compound of interest to be expressed and/or isolated. Typically, the culturing will comprise a growth phase mainly directed to formation of biomass and a production phase mainly directed to production of the compound of interest. The growth phase and production phase may overlap to some extent. A suitable nutrient medium comprises carbon sources, nitrogen sources and additional compounds (such as inorganic salts (e.g. phosphate), trace elements and/or vitamins) (see, e. g., Bennett, J. W. and LaSure, L., eds., More Gene Manipulations in Fungi, Academic Press, Calif., 1991) and can be performed under aerobic or anaerobic conditions.

The term “derived from” also includes the terms “originates from,” “obtained from,” “obtainable from,” “isolated from,” and “created from,” and typically indicates that one specified material finds its origin in another specified material or has features that can be described with reference to the another specified material. As used herein, a substance (e.g., a nucleic acid molecule or polypeptide) “derived from” a microorganism preferably means that the substance is native to that microorganism.

The term “expression” includes any step involved in the production of (a) polypeptide(s) including, but not limited to, transcription, post transcriptional modification, translation, post-translational modification, and secretion. A reduction or abolishment of production of B means a limitation to x % or less B produced via enzymatic conversion of A. This can be achieved with an enzyme/protein/cell/gene as described herein. An increase in production of B means an increase of at least x % B produced via enzymatic conversion of A compared to the amount B obtained in a process using a non-modified cell/wild type protein/enzyme/gene. Reduction or increase of gene expression can be measured by various methods, such as e.g. Northern, Southern or Western blot technology as known in the art. The terms “increase of activity” or “overexpression” are used interchangeably herein.

An expression cassette comprises a polynucleotide coding for a polypeptide, operably linked to the appropriate control sequences which allow for expression of the polynucleotide in a cell or in vitro.

The expression cassette may be an autonomously replicating vector (e.g plasmid), i. e., a vector the replication of which is independent of genome replication. Alternatively, the cassette may be one which, when introduced into the cell, is fully or partially integrated into the genome of the cell. In the latter cases it may comprise one or more targeting sequences to direct integration into the genome.

The expression cassette may or may not contain one or more selectable markers, which permit easy selection of transformed cells.

The term “polypeptide fragment” is defined herein as a polypeptide having one or more amino acids deleted from the amino and/or carboxyl terminus of the parent polypeptide.

The term “mature polypeptide” is defined herein as a polypeptide in its final form(s) and is obtained after translation of a mRNA into polypeptide, post-translational modifications of said polypeptide in or outside the cell. Post-translational modification include N-terminal processing, C-terminal truncation, glycosylation, phosphorylation and removal of leader sequences such as signal peptides, propeptides and/or prepropeptides as defined herein by cleavage.

The term “naturally-occurring” as used herein refers to processes, events, or products that occur in their relevant form in nature. By contrast, “not naturally-occurring” refers to processes, events, or products whose existence or form involves the hand of man. The term “non-naturally occurring” is herein synomymous with “man-made”. Generally, the term “naturally-occurring” with regard to polypeptides or nucleic acids can be used interchangeable with the term “wild-type” or “native”. It refers to polypeptide or nucleic acids encoding a polypeptide, having an amino acid sequence or polynucleotide sequence, respectively, identical to that found in nature. Naturally occurring polypeptides include native polypeptides, such as those polypeptides naturally expressed or found in a particular cell. Naturally occurring polynucleotides include native polynucleotides such as those polynucleotides naturally found in the genome of a particular cell. Additionally, a sequence that is wild-type or naturally-occurring may refer to a sequence from which a variant or a synthetic sequence is derived.

The term “nucleic acid construct” is herein referred to as a nucleic acid molecule, either single-or double-stranded, which is derived from a naturally occurring gene or which has been modified to contain segments of nucleic acid which are combined and juxtaposed in a manner which would not otherwise exist in nature. Nucleic acid constructs can be isolated, synthetically made of mutagenized. The term nucleic acid construct is synonymous with the term “expression cassette” when the nucleic acid construct contains all the control sequences required for expression of a coding sequence, wherein said control sequences are operably linked to said coding sequence.

The term “operably linked” as used herein refers to two or more components such as nucleic acid sequences or polypeptide sequences that are physically linked and are in a functional relationship with each other permitting them to function in their intended manner. For instance, a promoter is operably linked to a coding sequence if the promoter can regulate the transcription or expression of a coding sequence, in which case the coding sequence should be understood as being “under the control of” the promoter.

The term “parent polypeptide” refers to the polypeptide relative to which another polypeptide differs by substituting, adding or deleting one or more amino acids.

Position being defined with reference to SEQ ID NO: x means that the position in the amino acid sequence according to the disclosure at which a modification has taken place is given in respect with the position of the corresponding amino acid in the amino acid sequence according to SEQ ID NO: x when the two sequences are aligned using an alignment method as described herein.

The term “recombinant” when used in reference to a nucleic acid, or protein indicates that the nucleic acid, or protein has been modified in its sequence if compared to its native form by human intervention. The term “recombinant” when referring to a cell indicates that the genome of the cell has been modified in its sequence if compared to its native form by human intervention. The term “recombinant” is synonymous with “genetically modified”.

A selectable marker is a gene which allows for selection of cells transformed with such a gene and which provides for biocide or viral resistance, resistance to heavy metals, prototrophy to auxotrophs, and the like.

Preferred selectable markers include, but are not limited to, those which confer resistance to drugs or which complement a defect in the cell. They include e. g. versatile marker genes that can be used for transformation of most filamentous fungi and yeasts such as acetamidase genes or cDNAs, or genes providing resistance to antibiotics.

Alternatively, specific selection markers can be used such as auxotrophic markers which require corresponding mutant strains. Preferably, the selection marker is deleted from the transformed cell after introduction of the expression construct so as to obtain transformed cells which are free of selection marker genes.

The term selectable marker extends to a marker gene used for screening, i.e. marker gene that, once introduced into a cell confers to the cell a visible phenotype and causes the cell look different. An example of marker for screening is a gene coding for a fluorescent protein which causes cells to fluoresce under an appropriate light source.

For the purpose of this disclosure, it is defined here that in order to determine the percentage of sequence homology or sequence identity of two amino acid sequences or of two nucleic acid sequences, the sequences are aligned for optimal comparison purposes. In order to optimize the alignment between the two sequences gaps may be introduced in any of the two sequences that are compared. Such alignment can be carried out over the full length of the sequences being compared. Alternatively, the alignment may be carried out over a shorter length, for example over about 20, about 50, about 100 or more nucleic acids/based or amino acids. The sequence identity is the percentage of identical matches between the two sequences over the reported aligned region.

A comparison of sequences and determination of percentage of sequence identity between two sequences can be accomplished using a mathematical algorithm. The skilled person will be aware of the fact that several different computer programs are available to align two sequences and determine the identity between two sequences (Kruskal, J. B. (1983) An overview of sequence comparison In D. Sankoff and J. B. Kruskal, (ed.), Time warps, string edits and macromolecules: the theory and practice of sequence comparison, pp. 1-44 Addison Wesley). The percent sequence identity between two amino acid sequences or between two nucleotide sequences may be determined using the Needleman and Wunsch algorithm for the alignment of two sequences.

(Needleman, S. B. and Wunsch, C. D. (1970) J. Mol. Biol. 48, 443-453). Both amino acid sequences and nucleotide sequences can be aligned by the algorithm. The Needleman-Wunsch algorithm has been implemented in the computer program NEEDLE. For the purpose of this disclosure the NEEDLE program from the EMBOSS package was used (version 2.8.0 or higher, EMBOSS: The European Molecular Biology Open Software Suite (2000) Rice,P. Longden,I. and Bleasby,A. Trends in Genetics 16, (6) pp276-277, http://emboss.bioinformatics.nl/). For protein sequences EBLOSUM62 is used for the substitution matrix. For nucleotide sequence, EDNAFULL is used. The optional parameters used are a gap-open penalty of 10 and a gap extension penalty of 0.5. The skilled person will appreciate that all these different parameters will yield slightly different results but that the overall percentage identity of two sequences is not significantly altered when using different algorithms.

After alignment by the program NEEDLE as described above the percentage of sequence identity between a query sequence and a sequence of the disclosure is calculated as follows: Number of corresponding positions in the alignment showing an identical amino acid or identical nucleotide in both sequences divided by the total length of the alignment after subtraction of the total number of gaps in the alignment. The identity defined as herein can be obtained from NEEDLE by using the NOBRIEF option and is labeled in the output of the program as “longest-identity”.

The nucleic acid and protein sequences of the present disclosure can further be used as a “query sequence” to perform a search against public databases to, for example, identify other family members or related sequences. Such searches can be performed using the BLASTN and BLASTX programs (version 2.0) of Altschul, et al. (1990) J. Mol. Biol. 215:403-10. BLAST nucleotide searches can be performed with the BLASTN program, score=100, wordlength=12 to obtain nucleotide sequences homologous to nucleic acid molecules of the disclosure. BLAST protein searches can be performed with the BLASTX program, score=50, wordlength=3 to obtain amino acid sequences homologous to protein molecules of the disclosure. To obtain gapped alignments for comparison purposes, Gapped BLAST can be utilized as described in Altschul et al., (1997) Nucleic Acids Res. 25(17): 3389-3402. When utilizing BLAST and Gapped BLAST programs, the default parameters of the respective programs (e.g., BLASTX and BLASTN) can be used. See the homepage of the National Center for Biotechnology Information at http://www.ncbi.nlm.nih.gov/.

As used herein, the terms “variant,” “derivative”, “mutant” or “homologue” can be used interchangeably. They can refer to either polypeptides or nucleic acids. Variants include substitutions, insertions, deletions, truncations, transversions, and/or inversions, at one or more locations relative to a reference sequence. Variants can be made for example by site-saturation mutagenesis, scanning mutagenesis, insertional mutagenesis, random mutagenesis, site-directed mutagenesis, and directed-evolution, as well as various other recombination approaches. Variant polypeptides may differ from a reference polypeptide by a small number of amino acid residues and may be defined by their level of primary amino acid sequence homology/identity with a reference polypeptide. Preferably, variant polypeptides have at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or even at least 99% amino acid sequence identity with a reference polypeptide. Methods for determining percent identity are known in the art and described herein. Generally, the variants retain the characteristic nature of the reference polypeptide, but have altered properties in some specific aspects. For example, a variant may have a modified pH optimum, a modified substrate binding ability, a modified resistance to enzymatic degradation or other degradation, an increased or decreased activity, a modified temperature or oxidative stability, but retains its characteristic functionality. Variants further include polypeptides with chemical modifications that change the characteristics of a reference polypeptide.

With regard to nucleic acids, the terms refer to a nucleic acid that encodes a variant polypeptide, that has a specified degree of homology/identity with a reference nucleic acid, or that hybridizes under stringent conditions to a reference nucleic acid or the complement thereof. Preferably, a variant nucleic acid has at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or even at least 99% nucleic acid sequence identity with a reference nucleic acid. Methods for determining percent identity are known in the art and described herein.

The terms permease, facilitator, transporter or transport protein or related terms are all describing proteins with multiple membrane spanning domains that exhibit a function in transporting molecules across a membrane. This transport can be brought about by different mechanisms: uniport (transport of one molecule), symport (simultaneous co-transport of two different molecules in the same direction), antiport (simultaneous transport of two molecules in opposite directions), facilitated diffusion, group translocation (e.g. phosphotransferase systems) and primary transport (e.g. transport driven by hydrolysis of adenosine triphosphate or by light).

The family of sugar transporters in yeast can be divided in five clusters: hexose permeases (HXT-genes, GAL2), disaccharide permeases, myo-inositol permeases, sugar receptors and an additional cluster of transporters of which the substrate has been determined (Nelissen et al., 1997, FEMS Yeast Rev, 21, pp. 113-134).

In a first aspect, the invention provides a variant of a parent polypeptide which is preferably a hexose transporter, wherein the variant comprises an amino acid sequence which, when aligned with the amino acid sequence set out in SEQ ID NO: 1, comprises a substitution of amino acids N376 and T89, the positions of said amino acids being defined with reference to the amino acid sequence set out in SEQ ID NO: 1.

WO2014/195376 describes variants of hexose transporter polypeptides belonging to the Major Facilitator Superfamily (MFS), including Gal2 having substitutions at position N376 with increased preference towards xylose. However, WO2014/195376 is silent on the arabinose transport properties of N376 variants of Gal2 and does not disclose or suggest that substituting the threonine at position 89 to isoleucine results in a hexose transporter which has higher affinity for L-arabinose and which has lost all affinity for glucose (see Table 3).

The parent polypeptide is a member of the Major Facilitator Superfamily (MFS). Two main transporter families of which proteins are found throughout all living organism are the ATP-binding cassette (ABC) superfamily and the major facilitator superfamily (MFS), also known as the uniporter-symporter-antiporter family. Whereas ABC family permeases consist of multiple components and are primary active transporters, capable of transporting both small molecules and macromolecules only after generating energy through ATP hydrolysis, the MFS transporters consist of a single polypeptide of a secondary carrier which facilitates transport of small solutes in response to a chemiosmotic ion gradient. ABC superfamily and MFS proteins account for almost half of the solute transporters encoded within the microbial genomes (reviewed by Pao et al, 1998, Microbiol Mol Biol Rev.; 62 pp.1-34, and Saier et al, 1999, J Mol Microbiol Biotechnol, 1 pp.257-279).

The polypeptide according to SEQ ID NO: 1 is Gal2, which is a facilitated diffusion transporter. It belongs to the major facilitator superfamily, sugar transporter (TC 2.A.1.1) family. “Permease polypeptide”, is also designated herein as “polypeptide permease” or “polypeptide”. “Permease polypeptide polynucleotide”, is herein a polynucleotide that encodes the permease polypeptide.

In an embodiment, the variant polypeptide has xylose transport activity and/or arabinose transport activity, and/or galactose transport activity.

The variant polypeptide of the invention may have one or more alternative and/or additional activities other than that of sugar permease activity.

As set out above, a variant polypeptide of the invention will typically have sugar permease activity. However, a variant polypeptide of the invention may have one or more of the activities set out above in addition to or alternative to that activity.

The variant polypeptide is not necessarily a polypeptide according to SEQ ID NO: 1. Rather, the variant polypeptide comprises substituted amino acids, which, in SEQ ID NO: 1, are at positions 89 and 376. However, in a hexose permease polypeptide having another amino acid sequence than SEQ ID NO: 1, such positions may have a different numbering, but still correspond to positions 89 or 376 of SEQ ID NO: 1.

Positions 89 and 376, for (other) hexose permease polypeptides can be identified by comparing 3D crystal structures, or by aligning the amino acid sequences, as explained inter alia in WO2014/195376, in particular on page 17, line 22 and onwards. For example, in hexose transporter polypeptide Hxt11 (SEQ ID NO: 10), position N376 of the polypeptide of SEQ ID NO: 1 corresponds to N366 and position T89 corresponds to T79.

In an embodiment, the variant polypeptide comprises the amino acid sequence set out in SEQ ID NO: 1.

In another embodiment, the substitution of amino acid N376 comprises any one of N376T, N376C, N376V, N376M, N376L, N376I, or N376F. That is, the variant polypeptide has an amino acid at position 376 which is a threonine, a cysteine, a valine, a methionine, a leucine, an isoleucine, or a phenylalanine. Preferably position 376 is T, S, or I.

In an embodiment, wherein the substitution of amino acid T89 comprises any one of T89I, T89V, T89L, T895, T89M, or T89F.

That is, the variant polypeptide has an amino acid at position 89 which is an isoleucine, a valine, a leucine, a serine, a methionine, or a phenylalanine. Preferably, position 89 is an isoleucine.

In a preferred embodiment, the substitution of amino acid N376 comprises any one of N376T, N376S, or N376I, and the substitution of amino acid T89 comprises T89I.

In an embodiment, the variant polypeptide has at least 70% sequence identity with the amino acid sequence of the parent polypeptide or has at least 70% sequence identity of a polypeptide with an amino acid sequence corresponding to SEQ ID NO: 1. The variant may have at least 80% or 85% sequence identity with the amino acid sequence of the parent polypeptide or at least 80% or 85% sequence identity of a polypeptide with an amino acid sequence corresponding to SEQ ID NO: 1. The variant may have at least 90% sequence identity with the amino acid sequence of the parent polypeptide or at least 90% sequence identity of a polypeptide with an amino acid sequence corresponding to SEQ ID NO: 1. The variant may have at least 95% sequence identity with the amino acid sequence of the parent polypeptide or at least 95% sequence identity of a polypeptide with an amino acid sequence corresponding to SEQ ID NO: 1. The variant may have at least 98% sequence identity with the amino acid sequence of the parent polypeptide or at least 98% sequence identity of a polypeptide with an amino acid sequence corresponding to SEQ ID NO: 1. The variant may have at least 99% sequence identity with the amino acid sequence of the parent polypeptide or at least 99% sequence identity of a polypeptide with an amino acid sequence corresponding to SEQ ID NO: 1. The variant polypeptide is preferably a man-made polypeptide.

The invention further provides a nucleic acid sequence encoding the variant polypeptide of the invention, preferably wherein said nucleic acid sequence is man-made.

The invention also provides a nucleic acid construct comprising the nucleic acid sequence of the invention operably linked to one or more control sequences capable of directing the expression of the polypeptide variant in a suitable cell.

The invention also provides a recombinant expression cassette comprising the nucleic acid construct of the invention.

The invention also provides a recombinant yeast comprising the nucleic acid of the invention, the nucleic acid construct of the invention, and/or the expression cassette of the invention. The yeast is transformed with a nucleic acid sequence, a nucleic acid construct comprising said nucleic acid sequence, or a recombinant expression cassette comprising such nucleic acid construct. In an embodiment, such nucleic acid sequence encodes a variant polypeptide which is a mutant of a polypeptide that is native in the untransformed host cell. In an embodiment, the polypeptide that is native in the untransformed host cell is a transporter polypeptide chosen from the list consisting of Gal2, Hxt1, Hxt2, Hxt3, Hxt4, Hxt5, Hxt6, Hxt7, Hxt8, Hxt9, Hxt10, Hxt11, Hxt12, Hxt13, Hxt14, Hxt15, Hxt16 and Hxt17.

The recombinant yeast may be selected from Saccharomycetaceae, in particular from the group of Saccharomyces, such as Saccharomyces cerevisiae; Kluyveromyces, such as Kluyveromyces marxianus; Pichia, such as Pichia/Scheffersomyces stipitis or Pichia angusta; Zygosaccharomyces, such as Zygosaccharomyces bailii; and Brettanomyces, such as Brettanomyces intermedius, Issatchenkia, such as Issatchenkia orientalis and Hansenula/Ogateae.

The recombinant yeast may be subjected to evolutionary engineering to improve its properties. Evolutionary engineering processes are known processes. Evolutionary engineering is a process wherein industrially relevant phenotypes of a microorganism, herein the recombinant yeast, can be coupled to the specific growth rate and/or the affinity for a nutrient, by a process of rationally set-up natural selection. Evolutionary Engineering is for instance described in detail in Kuijper, M, et al, FEMS, Eukaryotic cell Research 5(2005) 925-934, WO2008/041840 and WO2009/112472. After the evolutionary engineering the resulting recombinant cell is isolated. The isolation may be executed in any known manner, e.g. by separation of cells from a recombinant cell broth used in the evolutionary engineering, for instance by taking a cell sample or by filtration or centrifugation.

In an embodiment, the recombinant yeast comprises one or more gene encoding an L-arabinose isomerase (E.C. 5.3.1.4; araA); one or more gene encoding an L-ribulokinase (E.C. 2.7.1.16; araB); and one or more gene encoding an L-ribulose-5-P4-epimerase (E.C. 5.1.3.4; araD); which genes allow said yeast to ferment arabinose. The genes for araA, araB and araD may be from Lactobacillus plantarum, as disclosed e.g. in WO2008/041840. Alternatively, an araA gene from Bacillus subtilis and araB and araD genes from Escherichia coli may be used as disclosed e.g. in EP1499708. Alternatively, araA, araB and araD genes may derived from of at least one of the genus Clavibacter, Arthrobacter and/or Gramella, in particular one of Clavibacter michiganensis, Arthrobacter aurescens, and/or Gramella forsetii, as disclosed in e.g. WO2009/011591.

In an embodiment the recombinant yeast comprises a gene encoding an L-arabinose isomerase having an amino acid sequence according to SEQ ID NO: 5 or a functional homologue thereof having at least 60%, preferably at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, at least 99% identity to SEQ ID NO: 5.

In an embodiment the recombinant yeast comprises a gene encoding an L-ribulokinase having an amino acid sequence according to SEQ ID NO: 6 or a functional homologue thereof having at least 60%, preferably at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, at least 99% identity to SEQ ID NO: 6.

In an embodiment the recombinant yeast comprises a gene encoding an L-ribulose-5-P4-epimerase having an amino acid sequence according to SEQ ID NO: 7 or a functional homologue thereof having at least 60%, preferably at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, at least 99% identity to SEQ ID NO: 7.

In one embodiment the recombinant yeast comprises one or more gene encoding a xylose isomerase (EC 5.3.1.5; such as xylA) and one or more gene encoding a xylulose kinase (E.C. 2.7.1.17; such as XKS1); or one or more gene encoding a xylose reductase (E.C. 1.1.1.307; such as XYL1 from Pichia/Scheffersomyces stipitis) and one or more genes encoding a xylitol dehydrogenase (E.C. 1.1.1.619; such as XYL2 from Scheffersomyces stipitis) to allow the recombinant yeast to ferment xylose.

In an embodiment the recombinant yeast comprises a gene encoding a xylose isomerase having an amino acid sequence according to SEQ ID NO: 8 or a functional homologue thereof having at least 60%, preferably at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, at least 99% identity to SEQ ID NO: 8.

In an embodiment the recombinant yeast comprises a gene encoding a xylulose kinase having an amino acid sequence according to SEQ ID NO: 9 or a functional homologue thereof having at least 60%, preferably at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, at least 99% identity to SEQ ID NO: 9.

A “xylose isomerase” (EC 5.3.1.5) is herein defined as an enzyme that catalyses the direct isomerisation of D-xylose into D-xylulose and/or vice versa. The enzyme is also known as a D-xylose ketoisomerase. A xylose isomerase herein may also be capable of catalysing the conversion between D-glucose and D-fructose (and accordingly may therefore be referred to as a glucose isomerase). A xylose isomerase herein may require a bivalent cation, such as magnesium, manganese or cobalt as a cofactor. Assays to measure activity of xylose isomerase and xylulose kinase are described inter alia in WO2003/062430.

In an embodiment the recombinant yeast further comprises overexpression of one or more genes of the non-oxidative branch of the pentose phosphate pathway.

In an embodiment the one or more genes of the pentose phosphate pathway that is overexpressed encodes for an enzyme selected from the list of a transaldolase (EC 2.2.1.2), a transketolase (EC 2.2.1.1), a ribose-5-phosphate isomerase (EC 5.3.1.6) and a D-ribulose-5-phosphate 3-epimerase (EC 5.1.3.1).

In another embodiment the one or more genes of the pentose phosphate pathway that is overexpressed is selected from the list of TALI, TAL2, NQM1, TKL1, TKL2, RPE1 and RKI1.

The invention also provides a method of designing a variant sequence of a parent polypeptide sequence, which method comprises:

a) selecting a parent polypeptide sequence, preferably a parent polypeptide with an amino acid sequence according to SEQ ID NO: 1;

b) substituting in said polypeptide sequence amino acids N376 and T89, the position of said amino acids being defined with reference to SEQ ID NO: 1.

In an embodiment, in step b) the substitutions of amino acids N376 result in any one of N376T, N376C, N376V, N376M, N376L, N376I, or N376F and the substitution of amino acid T89 results in any one of T89I, T89V, T89L, T89S, T89M, or T89F.

Preferably, the substitution of amino acid N376 results in any one of N376T, N376S, or N376I and the substitution of amino acid T89 results in T89I.

Techniques to design nucleic acids constructs encoding variant polypeptides, recombination expression cassettes, to transform yeast cells therewith, are described inter alia in WO2014/195376

The invention also provides a process for the production of a fermentation product, preferably ethanol comprising:

-   -   fermenting a composition comprising a lignocellulosic biomass,         in particular comprising glucose and arabinose, preferably also         comprising galactose, under anaerobic conditions in the presence         of a recombinant yeast according to the invention; and     -   recovering the fermentation product.

The fermentation product may be any of ethanol, butanol, lactic acid, di-terpene, glycosylated di-terpene, 3-hydroxy-propionic acid, acrylic acid, acetic acid, succinic acid, citric acid, fumaric acid, malic acid, itaconic acid, an amino acid, 1,3-propane-diol, ethylene, glycerol, a β-lactam antibiotic or a cephalosporin. Ethanol is a preferred fermentation product.

Anaerobic conditions are herein defined as conditions without any oxygen or in which essentially no oxygen is consumed by the recombinant yeast and usually corresponds to an oxygen consumption of less than 5 mmol/l.h, in particular to an oxygen consumption of less than 2.5 mmol/l.h, or less than 1 mmol/l.h. More preferably 0 mmol/L/h is consumed (i.e. oxygen consumption is not detectable. This usually corresponds to a dissolved oxygen concentration in the culture broth of less than 5% of air saturation, in particular to a dissolved oxygen concentration of less than 1% of air saturation, or less than 0.2% of air saturation. The process can also be carried out under micro-aerobic conditions.

In an embodiment the lignocellulosic biomass comprises lignocellulose and/or hemicellulose. The composition may be a biomass hydrolysate. Such biomass hydrolysate may be a lignocellulosic biomass hydrolysate. Lignocellulose herein includes hemicellulose and hemicellulose parts of biomass. Lignocellulose may also include lignocellulosic fractions of biomass. Suitable lignocellulosic materials may be found in the following list: orchard primings, chaparral, mill waste, urban wood waste, municipal waste, logging waste, forest thinnings, short-rotation woody crops, industrial waste, wheat straw, oat straw, rice straw, barley straw, rye straw, flax straw, soy hulls, rice hulls, rice straw, corn gluten feed, oat hulls, sugar cane, corn stover, corn stalks, corn cobs, corn husks, switch grass, miscanthus, sweet sorghum, canola stems, soybean stems, prairie grass, gamagrass, foxtail; sugar beet pulp, citrus fruit pulp, seed hulls, cellulosic animal wastes, lawn clippings, cotton, seaweed, trees, softwood, hardwood, poplar, pine, shrubs, grasses, wheat, wheat straw, sugar cane bagasse, corn, corn husks, corn hobs, corn kernel, fiber from kernels, products and by-products from wet or dry milling of grains, municipal solid waste, waste paper, yard waste, herbaceous material, agricultural residues, forestry residues, municipal solid waste, waste paper, pulp, paper mill residues, branches, bushes, canes, corn, corn husks, an energy crop, forest, a fruit, a flower, a grain, a grass, a herbaceous crop, a leaf, bark, a needle, a log, a root, a sapling, a shrub, switch grass, a tree, a vegetable, fruit peel, a vine, sugar beet pulp, wheat midlings, oat hulls, hard or soft wood, organic waste material generated from an agricultural process, forestry wood waste, or a combination of any two or more thereof. Lignocellulose, which may be considered as a potential renewable feedstock, generally comprises the polysaccharides cellulose (glucans) and hemicelluloses (xylans, heteroxylans and xyloglucans). In addition, some hemicellulose may be present as glucomannans, for example in wood-derived feedstocks. The enzymatic hydrolysis of these polysaccharides to soluble sugars, including both monomers and multimers, for example glucose, cellobiose, xylose, arabinose, galactose, fructose, mannose, rhamnose, ribose, galacturonic acid, glucuronic acid and other hexoses and pentoses occurs under the action of different enzymes acting in concert.

The lignocellulosic biomass may comprise pectin and/or other pectic substances such as arabinans, which may make up considerably proportion of the dry mass of typically cell walls from non-woody plant tissues (about a quarter to half of dry mass may be pectins). Lignocellulosic material may be pretreated. The pretreatment may comprise exposing the lignocellulosic material to an acid, a base, a solvent, heat, a peroxide, ozone, mechanical shredding, grinding, milling or rapid depressurization, or a combination of any two or more thereof. This chemical pretreatment is often combined with heat-pretreatment, e.g. between 150-220° C. for 1 to 30 minutes.

In another embodiment such composition is a pre-treated cornstover hydrolysate. Another preferred composition is a corn fiber hydrolysate, which is optionally pre-treated.

The composition preferably comprises glucose and arabinose, preferably also galactose. In an embodiment at least some of the glucose and at least some of the arabinose is consumed, preferably at least some of the glucose and at least some of the arabinose and at least some of the galactose is consumed.

The invention also provides the use of the recombinant yeast cell of the invention for the production of a fermentation product, preferably ethanol, from a composition comprising glucose, arabinose, and preferably also comprising galactose.

EXAMPLE Materials and Methods Strains and Maintenance

The construction of hexose transporter deletion S. cerevisiae strain DS68625 used for sugar uptake experiments in the example below was previously described in Shin et al., 2015 (An engineered cryptic Hxt11 sugar transporter facilitates glucose-xylose co-consumption in Saccharomyces cerevisiae, Biotechnology for Biofuels, volume 8, p. 176). The genotype of DS68625 is described as:

Mat a, ura3-52, leu2-112, gre3::loxP, loxP-Ptpi:TAL1, loxP-Ptpi::RKI1, loxP-Ptpi-TKL1, loxP-Ptpi-RPE1, delta::Padh1XKS1Tcyc1-LEU2, delta::URA3-Ptpi-xylA-Tcycl, his3::loxP, hxt2::loxP-kanMX-IoxP, hxt367::loxP-hphMX-loxP, hxt145::loxP-natMX-loxP, gal2::loxP-zeoMX-loxP

Stock cultures were grown in shake flasks on yeast-extract/peptone (YP) medium (10 g/L Bacto yeast extract, Becton Dickinson, Franklin Lakes, N.J., and 20 g/L Bacto Peptone, Becton Dickinson), This media was supplemented with 20 g/L maltose. Frozen stock cultures were prepared by adding glycerol (30% vol/vol) to stock cultures, after which 1 mL aliquots were stored at −80° C.

Cultivation and Media

Strain characterisation studies were performed in synthetic medium (SM) prepared as described previously (Verduyn C, Postma E, Scheffers W A, Van Dijken J P: Effect of benzoic acid on metabolic fluxes in yeasts: a continuous-culture study on the regulation of respiration and alcoholic fermentation. Yeast 1992, 8:501-517). Carbon source and vitamin solutions were added after autoclaving the medium for 20 min at 121° C. 50% w/v D-xylose solution was autoclaved separately at 110° C. for 20 min. Prior to inoculation 20 g/L D-xylose (SMX) or 20 g/L maltose (SMM) was added to SM as carbon source. SMX medium was used to propagate DS68625 derived transformants for sugar uptake assays. SMM medium was used in plates for selection after transformation.

Aerobic shake-flask cultures were grown in an orbital shaker at 200 rpm set at 30° C., using 500-ml flasks containing 100 ml medium. For plates 20 g/L agar (BD) was added prior to autoclaving at 121° C. for 20 min.

Plasmid and Strain Construction

The protein sequence for Gal2 originating from CEN.PK113-7D is given as SEQ ID NO: 1. The coding DNA sequence for GAL2 (YLR081W) found in CEN.PK113-7D is given as SEQ ID NO: 2 SEQ ID NO: 2 can be synthesized artificially at a DNA synthesis company and can be used as template for site-directed mutagenesis to generate specific Gal2 variants. The Gal2 variants tested in example below are given in Table 1.

TABLE 1 Amino acid changes in Gal2 variants Amino acid at Amino acid at Variant position 89 position 376 Gal2 (wt) T N Gal2^(N376I) T I Gal2^(N376S) T S Gal2^(N376T) T T Gal2^(N376T, T89I) I T Gal2^(T89I) I N

Plasmids constructed and used in this study are shown in Table 2. Plasmid DNA was isolated from E. coli cultures using a GenElute Plasmid kit (Sigma-Aldrich, St. Louis, Mo.). PCR amplification of expression cassettes and plasmid fragments was performed using Phusion High Fidelity DNA Polymerase (Thermo Scientific, Waltham, Mass., USA).

TABLE 2 GAL2 Expression plasmids Plasmid Characteristic Source pRS313-mcs CEN6/ARS4 HIS3 ampR pHXT7-mcs-tHXT7 Nijland et al., 2014* pRS313-Gal2 CEN6/ARS4 HIS3 ampR pHXT7-GAL2-tHXT7 This example pRS313-Gal2-TT CEN6/ARS4 HIS3 ampR pHXT7-GAL2^(N376T)-tHXT7 This example pRS313-Gal2-ST CEN6/ARS4 HIS3 ampR pHXT7-GAL2^(N376S)-tHXT7 This example pRS313-Gal2-IT CEN6/ARS4 HIS3 ampR pHXT7-GAL2^(N376I)-tHXT7 This example pRS313-Gal2-TI CEN6/ARS4 HIS3 ampR pHXT7-GAL2^(N376T/T89I)-tHXT7 This example pRS313-Gal2-NI CEN6/ARS4 HIS3 ampR pHXT7-GAL2^(T89I)-tHXT7 This example *Nijland J G, Shin H Y, de Jong R M, De Waal P P, Klaassen P, Driessen A J: Engineering of an endogenous hexose transporter into a specific D-xylose transporter facilitates glucose-xylose co-consumption in Saccharomyces cerevisiae. Biotechnol Biofuels 2014, 7: 168.

To construct expression plasmids, wildtype GAL2 and the abovementioned GAL2 variants were used as template and amplified using the primers F_Gal2_Xbal (SEQ ID NO: 3) and R_Gal2_Cfr9I (SEQ ID NO: 4). Resulting PCR fragments were subsequently cloned into plasmid pRS313-P7T7 similarly as described in Nijland et al., 2014 (Biotechnology for Biofuels, vol. 7, p. 168). All genes were cloned into pRS313-P7T7 and subsequently confirmed by Sanger sequencing (Baseclear, Leiden, The Netherlands).

S. cerevisiae strains were transformed as described by Gietz and Woods (Gietz R D, Schiestl R H, Willems A R, Woods R A: Studies on the transformation of intact yeast cells by the LiAc/SS-DNA/PEG procedure. Yeast 1995, 11). The Gal2 variants and the pRS313-P7T7-mcs plasmid (as an empty plasmid/control) were transformed to the hexose transporter deletion strain DS68625. Transformants were selected on SMM agar plates based on complementation by the GAL2 expression plasmids (which bear the HIS3 marker) of the histidine auxotrophy of DS68625. Resulting positive colonies were selected for each construct and named DS68625-Ga12, DS68625-Gal2 N3761, DS68625-Gal2 N376S, DS68625-Gal2 N376T, DS68625-Gal2 N376T/T89I, DS68625-Gal2 T89I and DS68625-mcs.

Sugar Transport Assays

Strains were pre-grown in aerobic shake flasks on SMX after which cells were collected by centrifugation (3000 g, 3 min), washed and re-suspended in SM without sugar. Uptake experiments were initiated by adding [¹⁴C] L-arabinose or [¹⁴C] D-glucose (ARC St. Louis, Mo., USA) to the cell suspension with varying concentrations between 0.2 and 500 mM. [¹⁴C] L-arabinose and [¹⁴C] D-glucose were added 50-60 mCi/mmol at a concentration of 0.1mCi/ml and labelled at the first carbon atom. At set time points, uptake was arrested by adding 5 mL of ice-cold 0.1 M LiCl, filtration over 0.45-μm HV membrane filters (Millipore, France) and washing with 5 mL ice-cold 0.1 M LiCl. Radioactivity on the filters was then counted using a Liquid Scintillation Counter (PerkinElmer, Waltham, Mass., USA) in Ultima Gold MV Scintilation cocktail (PerkinElmer, Waltham, Mass., USA). D-Glucose inhibition experiments were measured using 50 mM [¹⁴C] L-arabinose with [¹⁴C] D-glucose added at concentrations between 50 and 500 mM.

Results

The inhibitory effect of D-glucose on L-arabinose transport of positions N376 and T89 was tested in in S. cerevisiae DS68625. To examine the extent to which D-glucose inhibited L-arabinose uptake, [¹⁴C]-L-arabinose-uptake experiments were performed at different concentrations of D-glucose (FIG. 1). L-arabinose uptake by wild-type Gal2 was severely inhibited by D-glucose. The Gal2N^(376T) and Gal2^(N376T/T89I) substitutions significantly decreased inhibition of L-arabinose uptake by D-glucose, while D-glucose inhibition was not significantly affected by only the Gal2T⁸⁹I substitution (FIG. 1).

In transport assays with [¹⁴C]-L-arabinose or [¹⁴C]-D-glucose, Gal2^(N376I), Gal2^(N376S) and Gal2^(N376T) substitutions yielded lower K_(m) values for L-arabinose than wild-type Gal2, while their K_(m) values for D-glucose were one to two orders of magnitude higher than those of wild-type Gal2. As a result, the ratios of the K_(m) values for L-arabinose vs. D-glucose was 2 orders of magnitude lower for the strains expression Gal2 variants with substitutions at position 376. For Gal2 variants that only carried an amino acid substitution at this position, transport capacities (V_(max)) for L-arabinose and D-glucose differed by less than two-fold from those of wild-type Gal2 (Table 3). These changes in transport kinetics are consistent with a strongly reduced competitive inhibition of L-arabinose transport by D-glucose. A single Gal2T^(89I) substitution only slightly increased the K_(m) for D-glucose transport, while it decreased the K_(m) for L-arabinose 335 to 99 mM. For both sugars, the Gal2T^(89I) substitution caused a 2-3 fold reduction of V_(max) (Table 3). These kinetic properties suggest that Gal2T^(89I) may, by itself, confer a selective advantage at lower extracellular L-arabinose concentrations. Remarkably, a Gal2 variant that harboured both the Gal2^(N376T) and Gal2T^(89I) substitutions did no longer transport D-glucose, while K_(m) and V_(max) for L-arabinose were similar to those of Gal2T^(89I) (Table 3).

TABLE 3 K_(m) and V_(max) values for L-arabinose and D-glucose for Gal2 variants with amino acid substitutions at position N376 and T89. Transport kinetics were measured by uptake studies with radioactive sugars after expression of GAL2 alleles in S. cerevisiae DS68625. Values are average and mean deviation of two independent sets of uptake experiments. Data used to calculate K_(m) and V_(max) values are shown in FIG. 3. Km (mM) Km ratio Vmax (nmol (mg biomass)/min Gal2 variant L-arabinose D-glucose Ara/Glc L-arabinose D-glucose Gal2¹ 335 ± 21  1.9 ± 0.2 176 75 ± 5 26 ± 1 Gal2^(N376I) 117 ± 16 101 ± 47 1 39 ± 3  32 ± 18 Gal2^(N376S) 186 ± 33 38 ± 1 5 64 ± 2 28 ± 1 Gal2^(N376T) 171 ± 17 57 ± 1 3 65 ± 2 17 ± 4 Gal2^(T89I+N376T) 103 ± 40 — — 30 ± 2 *) Gal2^(T89I)  99 ± 18   7 ± 0.2 15 22 ± 3  13 ± 0.1 *) The detection limit for determining the d-glucose uptake velocity Vmax is 1.8 nmol⁻¹ (mg biomass)⁻¹ min⁻¹. 

1. A variant of a parent polypeptide which is optionally a hexose transporter, wherein the variant comprises an amino acid sequence which, when aligned with the amino acid sequence of SEQ ID NO: 1, comprises a substitution of amino acids N376 and T89, the positions of said amino acids being defined with reference to the amino acid sequence of SEQ ID NO:
 1. 2. A variant according to claim 1 wherein the parent polypeptide comprises the amino acid sequence of SEQ ID NO:
 1. 3. A variant according to claim 1, wherein the substitution of amino acid N376 comprises any one of N376T, N376C, N376V, N376M, N376L, N376I, or N376F.
 4. A variant according to claim 1 wherein substitution of amino acid T89 comprises any one of T89I, T89V, T89L, T89S, T89M, or T89F.
 5. A variant according to claim 1, wherein substitution of amino acid N376 comprises any one of N376T, N376S, or N376I, and wherein substitution of amino acid T89 comprises T89I.
 6. A variant according to claim 1 which has at least 80% sequence identity with the amino acid sequence of the parent polypeptide.
 7. A variant according to claim 1 which is a man-made polypeptide.
 8. A nucleic acid sequence encoding the variant according to claim 1, optionally wherein said nucleic acid sequence is man-made.
 9. A nucleic acid construct comprising the nucleic acid sequence of claim 8 operably linked to one or more control sequences capable of directing expression of the polypeptide variant in a suitable cell.
 10. A recombinant expression cassette comprising the nucleic acid construct of claim
 9. 11. A recombinant yeast comprising the nucleic acid sequence of claims 8, a nucleic acid construct comprising the nucleic acid sequence, and/or an expression cassette comprising said construct.
 12. A recombinant yeast according to claim 11 further comprising one or more genes encoding an L-arabinose isomerase (E.C. 5.3.1.4; araA); one or more genes encoding an L-ribulokinase (E.C. 2.7.1.16; araB); and one or more genes encoding an L-ribulose-5-P-4-epimerase (E.C. 5.1.3.4; araD).
 13. A recombinant yeast according to claim 11 further comprising one or more gene encoding a xylose isomerase (EC 5.3.1.5) optionally xylA and one or more gene encoding a xylulose kinase (E.C. 2.7.1.17) optionally XKS1.
 14. A recombinant yeast according to claim 11 further comprising overexpression of one or more genes of a non-oxidative branch of a pentose phosphate pathway.
 15. A recombinant yeast according to claim 14 wherein one or more genes of the pentose phosphate pathway that is overexpressed encodes an enzyme selected from a transaldolase (EC 2.2.1.2), a transketolase (EC 2.2.1.1), a ribose-5-phosphate isomerase (EC 5.3.1.6) and a D-ribulose-5-phosphate 3-epimerase (EC 5.1.3.1).
 16. A recombinant yeast according to claim 14 wherein one or more genes of the pentose phosphate pathway that is overexpressed is selected from TAL1, NQM1, TKL1, TKL2, RPE1 and RKI1
 17. A method of designing a variant sequence of a parent polypeptide sequence, which method comprises: a) selecting a parent polypeptide sequence, optionally a parent polypeptide with an amino acid sequence according to SEQ ID NO: 1; b) substituting said polypeptide sequence amino acids N376 and T89, the position of said amino acids being defined with reference to SEQ ID NO:
 1. 18. The method of claim 17, wherein in [[step]] b) the substitutions of amino acids N376 result in any one of N376T, N376C, N376V, N376M, N376L, N376I, or N376F and wherein the substitution of amino acid T89 results in any one of T89I, T89V, T89L, T89S, T89M, or T89F; optionally wherein substitution of amino acid N376 results in any one of N376T, N376S, or N376I and wherein substitution of amino acid T89 results in T89I.
 19. Process for the production of ethanol comprising: fermenting a composition comprising a lignocellulosic biomass, optionally comprising glucose and arabinose, optionally also comprising galactose, under anaerobic conditions in the presence of a recombinant yeast according to claim 11; and recovering the ethanol.
 20. Process according to claim 19 wherein the lignocellulosic biomass comprises lignocellulose and/or hemicellulose and/or pectin. 